Gamma-2 herpesviruses are associated with tumors in immunosuppressed hosts. Kaposi sarcoma, the most frequent malignancy in AIDS, correlates with infection by human herpesvirus 8 (KSHV, HHV8). To establish lifelong infections in immunocompetent individuals, it is essential for all herpesviruses to strike a balance between immune control and immune evasion. Major histocompatibility complex class I (MHC I) antigen presentation is frequently targeted by viral counter mechanisms to escape cytotoxic T cell control. Our long-term goal is to understand how pathogens thwart this pathway. Previously, we characterized evasion strategies used by alpha and beta herpesviruses. However, KSHV seems to dowregulate MHC I in a novel and unique way by expressing two viral proteins, K3 and K5, residing in the membrane of the endoplasmic reticulum. In contrast, MHC I molecules travel to the cell surface where they are rapidly internalized and degraded in intracellular vesicles. Several lines of evidence are consistent with a mechanism whereby the viral proteins modify the cytoplasmic tail of MHC I by transiently interacting in the ER. This post-translational modification, presumed to be ubiquitination, could then serve as an internalization signal at the cell surface. We will test this hypothesis by characterizing the structural motifs and post-translational modifications of MHC I molecules and by studying their interaction with the viral proteins. The aminoterminal region of K3 and K5 contains a conserved PHD/LAP finger that is essential for MHC downregulation. Current evidence suggests that the PHD-domains interact with cellular proteins. Using several different aprpoaches, we will search for these cellular interacting partners. Open reading frames, homologous to K3 and K5, can be found across the gamma-2 herpesvirus family. Importantly, the homologous gene, K3, of murine gamma herpesvirus 68 (MHV68) was also shown to downregulate MHC I. Moreover, K3-deleted virus is deficient in maintaining latency. This model can thus be used to explore if KSHV-K3 or K5 can restore pathogenesis of K3-deleted MHV68. We expect the results of our experiments to have an impact on the understanding of the cell biology, immunology and pathogenesis of infections by gamma-2 herpesviridae.